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Apple Hard Disk SC Capacitor Replacement Guide

From RetroTechCollection
The power supply (item 5) sits beside the hard disk assembly in the case shared by the 20SC, 40SC, 80SC and 160SC

The Apple Hard Disk 20SC, 40SC, 80SC and 160SC all use the same power supply, Apple part 661-0343.[1] It is a switch-mode supply giving 5 V and 12 V to the drive and the 12 V fan, and it runs from any mains voltage between 85 and 270 V AC without a selector.[2][3] Apple serviced the supply as a module and published no schematic or component list for it.[1]

Warning: the supply's primary side runs at mains voltage, and Apple warns that the drive's high-voltage components keep their charge after switch-off.[4] Unplug the drive and discharge the high-voltage capacitors before working on the board.

Capacitor list

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The only published list is from Adam Wilson's 2023 rebuild of the supply from a 1988 20SC. He found no ready-made kit, so he noted every electrolytic on the main board and its daughterboard and replaced all of them with 105 °C parts from known brands. He did not publish board positions.[2]

Electrolytic capacitors in one Hard Disk 20SC power supply (community list, Adam Wilson)[2]
Quantity Capacitance Voltage Type
1 4.7 µF 350 V Radial
1 22 µF 35 V Radial
1 22 µF 100 V Radial
2 47 µF 25 V Radial
2 100 µF 10 V Radial
1 150 µF 400 V Radial snap-in
1 330 µF 16 V Radial
1 470 µF 10 V Radial
2 2200 µF 16 V Radial

The list covers twelve capacitors from a single unit. Apple used one part number for the supply across the range, but this is not proof that every supply has the same board, so check the values printed on your own capacitors against the table before ordering.[1][2]

The same supply's daughterboard had two 3.3 Ω 1 W resistors that had been damaged by overheating, and they were replaced along with the capacitors.[2] Check those resistors for discoloured bodies or a scorched board while the supply is apart.

Procedure

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  1. Remove the case top and lift out the power supply, disconnecting the drive power connector and the fan cable. The steps are on Apple Hard Disk SC General Maintenance.[1]
  2. Remove the two screws holding the supply's lid.[2]
  3. Release the board, which is held by its mains lead and four screws, two on the bottom and two on the side.[2]
  4. Photograph both boards before removing anything. Adam Wilson warns that board silkscreen is not always right, so record the polarity of each original capacitor.[2]
  5. Replace the capacitors one at a time, with the same capacitance and the same or a higher voltage rating.[2]
  6. Clean off flux with isopropyl alcohol.[2] Refit the board and lid, and test the supply in the drive before closing the case.

After the rebuild, Adam Wilson's supply ran the fan and the BlueSCSI adapter he fitted in place of the failed drive.[2]

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References

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  1. ↑ 1.0 1.1 1.2 1.3 Apple Computer, Apple Service Technical Procedures, Cross Family Peripherals, Volume One (PN 072-0230, 1991), SCSI Hard Disk Drives chapter, hosted as File:Apple Service Technical Procedures Cross Family Peripherals Volume One Apr 1991.pdf. Take-Apart, pp. 2.6-2.7 (PDF pp. 192-193): removing the power supply; Illustrated Parts List, IPL.3 (PDF p. 256): one power supply, 661-0343, for the complete assembly.
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 Adam Wilson, "1988 Apple 20SC External HDD Repair & Restoration", Adam's Vintage Computer Restorations, 25 January 2023, retrieved 2026-10-04. Source for the supply construction (5 V/12 V switch-mode, lid held by two screws, board held by its mains lead and four screws), the capacitor list, the two overheated 3.3 Ω 1 W resistors and the replacement method.
  3. ↑ Apple Computer, "Apple Hard Disk 20SC: Specifications (Discontinued)", Tech Info Library article 1931, revised 13 September 1993, in the archive.org collection Apple's Technical Information Library Articles, https://archive.org/details/APPLE_TIL_ARTICLES, file TIL00001_to_TIL60496.zip. Line voltage 85-270 V AC, adapting automatically; 60 W.
  4. ↑ Apple Computer, Apple Hard Disk 20SC Owner's Guide (030-3138-A, 1986), printed p. 22 (PDF p. 32), hosted as File:Apple Hard Disk 20SC Owners Guide 1986.pdf.


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